1. Field of the Invention
This invention relates to charge coupled devices and in particular to a charge coupled imaging device in which the output signal is detected and amplified by a pair of floating gate preamplifiers separated by a charge limiting well. The first preamplifier may be adjusted for optimum detection of high level signals while the second operates on lower level signals. The charge limiting well prevents high level signals from reaching the second preamplifier.
2. Prior Art
Many types of charge coupled linear and area imaging devices are now well known. Such devices are readily commercially available and have been the subject of numerous patents. For example, Fairchild Camera and Instrument Corporation manufactures a wide variety of linear and area imaging devices ranging in size from product CCD111, a 1X256 linear imaging device, to produce CCD221, a 380.times.488 area imaging device.
The development of large area high resolution image sensors has been hindered, however, by the widely varying ambient lighting conditions such devices must sense. Such ambient lighting causes widely disparate amounts of charge to accumulate in different elements of the charge coupled image sensor. Prior to this invention sense amplifiers have not been able to efficiently detect and amplify disparate amounts of charge resulting from such different light levels.
Floating gate and distributed floating gate amplifiers for charge coupled image sensing are known. These operate by sensing the size of a signal charge packet by the image charge created in the electrically isolated, but proximate, floating gate electrode. Because floating gates sense the signal charge nondestructively, floating gate amplifiers can be distributed periodically along an output register to repeatedly detect the signal charge traveling through the register. This enhances the signal to noise ratio, but does not improve the ability of the amplifier to process widely varying signals accurately. Thus, floating gate amplifiers typically have provided a linear response over a large but sometimes inadequate range of charge densities. For example, the linear response of the floating gate amplifier in Fairchild product CCD221 accurately senses signals varying by four orders of magnitude. A brief summary of floating gate amplifier operation in charge coupled image sensors may be found beginning at page 56 of Charge Transfer Devices by Sequin and Tompsett, Academic Press, 1975.